The present invention relates to an acceleration sensor, and more particularly to an automobile air bag control apparatus using an acceleration sensor.
One known acceleration sensor formed by layering glass or silicon is described in JP-A-3-134570.
JP-B-4-55267 (equivalent to U.S. Pat. No. 4,679,434) discloses an acceleration sensor including a flat substrate having mounted thereon an acceleration detecting element and an electronic circuit for detecting a change in electrostatic capacitance of the acceleration detecting element. The substrate is mounted in a metal housing. The acceleration sensor is fixed in place by means of screws to be inserted through mounting holes formed through metal flanges in the housing.
The direction of acceleration to be detected by the acceleration sensor is perpendicular to the mounting surface of the acceleration sensor. Further, the use of the metal housing makes the acceleration sensor heavy. This structure results in various limitations on the mounting of the acceleration sensor in a system using the sensor.
In an air bag system, for example, a control unit of the air bag system is usually mounted in a horizontal direction of a vehicle, whereas the direction of acceleration to be detected by the acceleration sensor is the same as a longitudinal direction of the vehicle.
Accordingly, in such an application, because of the fact that the direction of acceleration to be detected by the acceleration sensor is perpendicular to the mounting surface of the acceleration sensor, it is necessary to mount the sensor perpendicular to the longitudinal direction of the vehicle. This necessitates complex mechanical arrangements for the mounting of the sensor.
Moreover, because of the relatively large weight of the acceleration sensor itself, the sensor is liable to mechanically resonate (which causes an error in detection of the acceleration).
JP-A-5-340963 discloses an acceleration sensor in which the acceleration detecting element is also mounted on a base plate such that the direction of acceleration to be detected is perpendicular to the base plate. The base plate is itself however mounted perpendicular to the metal base of the acceleration sensor housing, such that the direction of acceleration to be detected by the sensor is parallel to the mounting surface of the sensor. However, this construction still results in a complex and bulky device.
EP-B-0369352 discloses a capacitance-type accelerometer comprising three parallel silicon plates, the central one of which includes a movable electrode part. The plates are insulated from each other using thermal-oxide films in all areas between the plates except those adjacent the movable electrode. There is no discussion of a method of mounting the accelerometer.
It is an object of the present invention to provide an acceleration sensor which mitigates some of the above disadvantages.
In a first aspect the present invention provides an acceleration sensor including a base and an acceleration detection element of capacitance type mounted on the base. The acceleration detection element has a movable electrode mounted adjacent a fixed electrode such that acceleration of the sensor in a measurement direction causes the movable electrode to move relative to the fixed electrode. The element further has opposite ends in a direction perpendicular to the measurement direction, and the acceleration detection element is mounted on the base at a first one of the opposite ends.
The acceleration detection element being mounted at a first one of the opposite ends means that the first one of the ends is directed generally towards the base. The end need not necessarily contact the base and in some embodiments the end is separated from the base by bonding material, such as an elastomeric adhesive, used to bond the acceleration detection element to the base.
Thus, the acceleration detecting element of the acceleration sensor is arranged so that the direction of acceleration to be detected by the acceleration sensor is parallel to a mounting surface of the acceleration sensor. Accordingly, the direction of acceleration to be detected by the acceleration sensor mounted on the mounting surface is parallel to the surface, thereby making the mounting of the acceleration sensor easier.
Further, the acceleration detecting element may be airtightly sealed by a mounting surface or substrate on which the acceleration detecting element is mounted, and a cover mounted on the substrate so as to surround the acceleration detecting element, thereby reducing the weight of the acceleration sensor without use of a metal housing. Accordingly, even when the acceleration sensor is fixed to a printed board having a low mechanical rigidity, no resonance of the acceleration sensor occurs.
The element may comprise a movable electrode mounted between a pair of fixed electrodes, with the element preferably having at least one end face which is substantially parallel to the direction of acceleration detectable by motion of the movable electrode, wherein the element is mounted on the base such that the end or end face lies on the base. The end face may be separated from the base by e.g. a fixing medium such as an adhesive.
Preferably, the movable electrode is supported by a flexible beam which extends from the movable electrode towards the other of the opposite ends.
By locating the beam end of the movable electrode away from the base the effect of vibrations on the acceleration detection element are reduced.
Preferably, the element has a pair of end faces which are each substantially parallel to the direction of acceleration detectable by motion of the movable electrode. A first one of the pair of end faces is the most remote from the beam, and the element is mounted on the base such that the first end face lies on the base.
Preferably, the acceleration detection element includes at least one electrical connection terminal located at the other of the opposite ends.
Preferably the at least one electrical connection terminal for electrical connection to the acceleration detection element is located in an end region of the acceleration detection element remote from the end or face which lies on the plate. Preferably the sensor includes at least first and second electrical connection terminals for electrical connection to the movable electrode and the fixed electrode respectively, and possibly a third electrical connection terminal for connection to a second fixed electrode.
The above arrangements provide relatively easy access to the terminals and allow for easier connection of e.g. wires thereto.
The acceleration detection element may be attached to the base by elastomeric adhesive, such as silicone rubber. This provides a relatively secure attachment whilst also giving some insulation of the acceleration detection element from external vibrations.
Preferably, the base is a ceramic base such as a ceramic plate and the sensor further includes signal processing means mounted on the base, the signal processing means being electrically connected by electrical connection means to the acceleration detection element, and the signal processing means being adapted to convert electrical information output from the element into an electrical signal related to acceleration detected by the element.
Advantageously, the ceramic base includes therein or thereon the electrical connection means. In other words, the ceramic base may also be a circuit board having tracks or other circuit elements located thereon. Preferably, the electrical connection means connect the signal processing means to the movable electrode and the fixed electrode.
Preferably, the element includes at least first and second electrical connection terminals for electrical connection to the movable electrode and the fixed electrode respectively, and the terminals are connected using solder directly to the electrical connection means on the base.
In a second aspect, the present invention provides electrical apparatus including an acceleration sensor as described above mounted on a circuit board using mounting means, wherein the mounting means also provide electrical connection between the circuit board and the acceleration sensor.
In this way, a more compact packaging for the acceleration sensor included in the electrical apparatus may be provided. Such a packaging may, for example, be a leadless chip carrier.
Preferably, the acceleration sensor is mounted on the circuit board such that the base (e.g. a base plate) is substantially perpendicular to the circuit board. In this way the direction of acceleration detection may be chosen to be either parallel or perpendicular to the circuit board as required.
In one embodiment, the movable electrode comprises a first semiconductor layer which is mounted between first and second insulating layers, there being a pair of the fixed electrodes provided respectively on faces of the first and second insulating layers adjacent the movable electrode. The acceleration sensor further includes second and third semiconductor layers each respectively mounted on a face of the first and second insulating layers remote from the movable electrode, with each of the fixed electrodes being connected to a different one of the second and third semiconductor layers.
According to a third aspect, the present invention provides an acceleration sensor including an acceleration detection element mounted on a ceramic base, and signal processing means mounted on the base and electrically connected by electrical connection means to the acceleration detection element. The base includes electrical tracks being part of the electrical connection means and the signal processing means are adapted to convert electrical information output from the element to an electrical signal related to acceleration detected by the element.
In this aspect, the invention is applicable to sensors having acceleration elements of various different types, such as a strain gauge type and a piezoelectric type, but it is especially applicable to the capacitative type.
According to a fourth aspect, the present invention provides an automobile air bag control system including an acceleration sensor or electrical apparatus according to any of the above aspects.